Conventional arc lamps of this type, in general, have a hole through a metal plate positioned in the upper part of an arcing chamber defined by the metal plate and a glass globe, and an upper carbon electrode extends through said hole. The upper carbon electrode is held by a holder above the metal plate, and the upper carbon electrode holder moves up and down along guide rods provided on the right and left sides thereof. The upper electrode passes through a ring clutch which in turn is actuated by an electromagnetic control device which is connected with said ring clutch in order to control the up and down movement of the upper carbon electrode. In such prior art devices, the upper carbon electrode must be electrically insulated from the metal plate where it passes through the hole.
A lower carbon electrode is held by a lower carbon electrode mounting which is attached to a rod suspended from said metal plate. The carbon electrodes are replaced by detaching the globe, and for replacing the upper carbon electrode, the electrode is inserted through a hole in the lower carbon-electrode holder, passed through the metal plate and a double ring of the clutch, and then is fastened to the upper carbon electrode holder.
While arcing is occurring in the lamp, the lower carbon electrode remains stationary, but the upper carbon electrode is controlled in its up and down movement by the control device which operates in accordance with the arcing current or voltage. The ring clutch which is actuated by said control device has a double ring construction, one side of the upper ring being fastened by a hinge to the corresponding side of the lower ring, so that when the other side of the upper ring is pulled up, the edge of the upper ring will bite into the carbon electrode, and upon further upward movement of the ring clutch, the carbon rod will be pulled up. To lower the carbon electrode, the upper ring biting into the carbon electrode is lowered, so that the carbon electrode can slide down through the two rings of the clutch, and the downward movement is stopped at an appropriate position by pulling up the said other side of the upper ring.
A lot of carbon rods are required for extended periods of testing, as the arc discharged during a day consumes a whole carbon rod. It is also necessary that the carbon rod have a uniform diameter and be free of bends, and have a rugged surface. However, it is technically difficult and expensive to make carbon rods having a uniform diameter and uniform surface conditions. If the diameter is not uniform, the ring does not completely bite into the carbon rod and cannot be moved up-and-down smoothly, giving rise to frequent short-circuiting and extinction of the arc. Therefore, it has been customary to grind the edge of the rings of the ring clutch. But even if the edge of the rings on the ring clutch are ground, variations in the diameter of the carbon rod or excessive bending of carbon rod interrupts the smooth up-and-down movement, often resulting in sliding down of the carbon rod. The ring clutch is also disposed at a position which is hard to see from outside the lamp. Therefore, it is very hard, after the carbon rod is passed upwardly through the metal plate during replacement of the rod, to make sure whether the carbon rod is running through the ring clutch. In particular, since hot air heated by the metal plate is directly emitted from the testing apparatus, the lamps which are enclosed make it less feasible to make sure whether the carbon rod has been passed through the ring clutch properly. Thus, the carbon rod has often not been properly passed through the ring clutch, resulting in unexpected accidents.